This invention relates to methods and apparatus for programming cellular programmable logic integrated circuits such as field programmable gate arrays ("FPGAs") and many kinds of programmable logic devices ("PLDs").
Extremely powerful and flexible cellular programmable logic circuit architectures are known as shown, for example, by commonly assigned, co-pending application Ser. Nos. 754,017, filed Sep. 3, 1991, and 880,942, filed May 8, 1992, both of which are hereby incorporated by reference herein. These architectures include large numbers of logic modules, each of which is programmable to perform any of several relatively elementary logic functions. An extensive network of conductors is provided for programmably interconnecting these logic modules in order to provide much more complex logic functions. These logic circuits include very large numbers of programmable elements. Each logic module has a substantial number of these elements, and the interconnection network also requires many such elements to produce the desired interconnections between logic modules.
A typical technique for programming the programmable elements in devices of the type described above is to employ shift registers as shown, for example, in Wahlstrom U.S. Pat. No. 3,473,160 and Freeman U.S. Pat. No. 4,870,302. Each shift register stage controls an associated logic or switching element. Programming data is shifted through the shift register or registers until the data desired for controlling each logic or switching element is stored in the shift register stage associated with that element. A disadvantage of this approach is that shift registers are relatively complex and require substantial numbers of conductors for interstage data transfer, clocking, etc. Thus in complex logic circuit structures, the shift register approach to programming may take up excessive space and other resources, and may even become a limiting factor in the design of the device. The testing of shift register programmed devices is also relatively cumbersome because the shift register can only be tested by shifting data all the way through it. If, as is common in very complex logic circuits, the shift registers are long, it may take a relatively long time to shift test data through them.
In view of the foregoing, it is an object of this invention to provide improved techniques for programming cellular programmable logic circuits.
It is a more particular object of the invention to simplify the structures used for programming cellular programmable logic circuits.
It is still another more particular object of this invention to provide cellular programmable logic circuit programming techniques which facilitate testing of the device.